1. Field of the Invention
This invention relates to seismic data processing and particularly to a method for identifying and removing boundary-generated noise artifacts manifested in both actual and synthetic seismic data.
2. Description of the Related Art
Seismic data generally consist of a plurality of traces plotted as a function of time or depth versus location along the surface of the earth. The data may be from a field location, or it may consist of synthetic data derived from a geologic model. Each trace displays the subsurface response to an acoustic signal. These responses aid the geologist and geophysicist to interpret the structures present in the subsurface. To obtain a meaningful and accurate display, the data are often reprocessed. The data are manipulated in a data processing center using both manual and digital computer techniques to provide the best section. Such manipulations typically involve moving adjacent traces up or down to correct for topographic or near surface irregularities, normal move-out (NMO), and stacking.
Once the data are corrected for statics, normal move-out and stacking, the data are often filtered. Band-pass filtering is a process for eliminating the frequencies that are not seismically useful; typically frequencies above or below those transmitted by the subsurface layers. When the seismic energy is transmitted through the earth, distortions may be induced which may be removed by a filtering process called convolution.
Lastly, it is a generally known that the subsurface layers are not always horizontal, but may dip or be folded. Seismic energy reflected from such dipping or folded layers also introduce errors in the data. Seismic data displayed in a seismogram are displayed vertically, which incorrectly displays the location of reflections from dipping or folded subsurface layers. To correctly position these reflections, the paths of the seismic signals must be changed during the processing step. Migration is the process employed to make these corrections. If the subsurface layers dip more than about 10 degrees, un-migrated sections can be misleading. For example, anticlines may appear much larger than in fact. Several types of migration are available to the operator to make the corrections for processing purposes, each type changing the domain in which the data were originally collected.
After data processing, including the static, dynamic and residual corrections, band-pass filtering, deconvolution and migration, the seismogram generally is ready for interpretation. However, the seismogram is not in its best possible form. Certain noise artifacts are difficult to remove using the above processing techniques. One type of artifact is generated during migration processing and is known as boundary-generated noise. Boundary-generated noise consists of diffraction hyperbola or semicircles appearing at the boundaries of the seismic grid defining the seismogram. The data processing algorithms used to originally process the raw data may assume that the data is periodic: in essence the section is repeated as a function of time. The algorithms in the data processing routine recognize the artificial boundaries as point sources of energy and try to manipulate them as it would the rest of the seismic data. The end result are noise artifacts such a semicircles or tails of semicircles appearing in the final section.
In the past, boundary-generated noise was removed by adding zeros to the end of the data prior to the processing stage. This process is termed "padding" or "zero padding." A major problem associated with zero padding during the initial processing stage is the increased computer time necessary to process the data. Computer (CPU) time is generally expensive and is a leading contributor to seismic exploration costs. For the sake of example, assume the cost to process data without zero padding and leaving the boundary-generated noise in the section can be equated to one. The cost to conventionally process the same seismic data padded with zeros for removing the boundary-generated noise may be four times greater.
There has been a long felt and yet unsolved need for a method to remove this type of artifacts from the seismogram while simultaneously reducing data processing costs over the conventional technique.